1. Field of the Invention
This invention relates to an apparatus for applying liquid toner to develop an electrographic latent image formed on an image bearing, charge retaining surface of a recording medium moving along a predetermined axis of travel with respect to the toner applicator.
More particularly, this invention relates to an improved toner applicator structure defining an internal reservoir of toner and having a series of continuous parallel slots disposed for developmental engagement across the full width of the moving recording medium for applying a uniform flow of toner to develop the electrographic latent image. The toner applicator is also articulated on a simple flexure mount which maintains a more precisely controlled and constant development gap between the applicator and the surface of the recording medium than was previously possible. The flexure mounted applicator is also adapted to achieve substantially simultaneous and complete separation of the toner applicator away from the surface of the recording medium. This improves image quality by immediately breaking the meniscus of toner formed at the development gap and facilitates drying of the toner.
In a conventional electrographic writing system, the electrographic latent image formed on the image bearing, charge retaining surface of a recording medium is developed and made visible by the application of a liquid toner. In the prior art, an applicator roller is typically submerged in a liquid developer bath provided in a receptacle. In some cases, the applicator roller may be rotated at a sufficiently high velocity so as to raise liquid developer from the bath in the form of a toner film on the surface of the roller due to viscous friction. A moving recording medium is brought into a predetermined proximity with the surface of the applicator roller to form a toning or development gap between the roller and the recording medium.
A meniscus of toner fluid typically is maintained in the development gap by reason of the application of an electric field across the development gap or a bias applied to the applicator roller. As the recording medium moves along an axis of travel past the toning station, the applicator roller is brought into a predetermined tangential developmental engagement with the surface of the recording medium. As the applicator roller rotates, the liquid toner is applied to the latent electrostatic charged image causing it to become visible. Subsequently, along the axis of travel of the recording medium, a wringing roller or drying roller is provided for engaging the recording medium and removing the excess toner which may have remained on the surface of the recording medium. Typically, a drying roller is driven to rotate in a counter direction to the direction of travel of the recording medium.
In conventional electrographic plotters and printers or the like using liquid toner, the toner applicator must be pre wet in a toning reservoir before it is used since it often dries out between each use. The drying out of a toner applicator roller is a serious problem in the prior art. Toner applicators must be periodically cleaned to remove toner particles which become compacted or dried onto the surface of the applicator. Due to the need for maintaining the extremely precise alignment of the toner applicator, the cleaning must be done by hand using large amounts of a cleaning solvent. This is particularly disadvantageous since cleaning the applicator roller often must be done as frequently as once every day. This results in high maintenance costs and much lost time during which the electrographic plotter, printer or the like cannot be used.
In some prior art apparatus for developing an electrostatic latent image, a negative or positive bias is applied to the applicator roller in order to force the entrained toner particles in the liquid developer on the roller to be attracted toward the surface of the roller against the inertia force tending to drive the toner particles toward the recording medium surface. Although this method supposedly provides a more uniform application of toner by preventing the formation of an uneven density of toner particles in the development meniscus, applying a bias to the applicator roller has certain disadvantages. The application of an electric bias to an applicator roller causes the toner to "plate up" in a manner analogous to electroplating and keeps toner particles firmly adhered to the roller. In many cases, if the toner applicator is not cleaned constantly, the plating up of toner particles is so severe that they cannot be removed from the applicator roller. This results in an undue amount of maintenance and a shortened useful life for an applicator roller. The continual accumulation or building up of toner on the biased applicator roller also adversely affects the toner applying capability of the roller as well as the density and color quality of the developed electrographic image.
Another problem encountered in prior art toner applicator systems is the inability to achieve instantaneous removal of the development meniscus when the plot or development of the latent electrographic image is to be concluded. Typically, the toner is applied to a latent electrostatic image on the recording medium through the establishment of a meniscus of toner in a development gap between the applicator roller and the moving recording medium. In the prior art, this gap typically may be on the order of at least 0.020 inches. If the movement of the recording medium is interrupted for any reason, stains or stopping marks may appear on the developed image due to the inability to instantaneously remove the meniscus from the development gap. That is, the meniscus always remains for a short period of time in the development gap at the end of a plotting operation and continues to wick toner from the applicator roller into the surface of the paper.
One prior art method for preventing the toner from wicking into the paper is described in U.S. Pat. No. 4,454,833. This patent uses an elastomeric flap which rests against the applicator roller and creates a closed path for the toner between the applicator roller and recording medium. However, the elastomeric flap is somewhat fragile and therefore is subject to extreme wear which results in deteriorating image quality. In another embodiment, U.S. Pat. No. 4,454,833 uses the previously discussed method of applying a bias to the applicator roller to cause entrained toner particles in the liquid developer to be attracted toward the surface of the roller. While this may aid in preventing a high density of toner particles in the meniscus, and thus alleviate wicking into the paper, it has the disadvantage of causing toner plating or build up on the applicator roller. Because the toner particles are electrically plated on to the roller, they cannot be removed easily. The plating up of toner particles on the roller deteriorates the image quality which can be produced by the roller and results in high maintenance costs and a shortened useful life.
An attempted solution to the problem of preventing the meniscus in the development gap from wicking toner into the paper and impairing image quality is to eliminate the development gap altogether. U.S. Pat. No. 4,733,273 attempts to eliminate the development gap by using an applicator roller to form a seal with a recording medium and to provide a so called "development apex" forward of the seal.
However, this method suffers from several drawbacks. For example, it is difficult to form a sealing barrier between a moving recording medium and a roller surface. Also, the development apex must be very precisely maintained with the surface of the recording medium for proper image quality. If the distance is too great, the development apex will not be established. If the distance is too narrow, the established development apex will be difficult to break up, even when the toner pump is terminated. This may cause the wicking of excess toner onto the surface of the recording medium along with concomitant staining and image degradation.
In the prior art toner applicator systems which employ an applicator roller, the roller usually is not independently articulated with respect to the recording medium. Typically, there is no provision for separating the applicator roller away from the surface of the recording medium when a plot is concluded or upon unintended interruption of the movement of the recording medium.
Moreover, prior art articulating systems may be mechanically complex or use sliding bearings. These systems always have an inherent amount of friction and backlash which must be overcome when it is necessary to break the toning meniscus. A sliding bearing articulation system is subject to wear and is prone to lose its ability to maintain the precise operation necessary for good image quality. For example, if a development meniscus is not maintained with extreme precision, excess toner may either smear or take a long distance to dry on the recording medium. This has the disadvantage of degrading image quality or using up valuable plotting space on the recording medium.
Another problem encountered in prior art electrographic plotters and printers using liquid toner and a drying roller disposed downstream from the toner applicator arises from the constant yet almost imperceptible buildup of toner particles and paper fibers on the interface between the drying roller and the scraper blade. The scraper blade is typically disposed on the underside of the drying roller for contacting the roller across its entire width and for scraping excess ink off of the roller so it does not affect the next writing past. However, in the course of normal operation the roller picks up a coating of paper fibers and toner particles which may not be visible. Also, the excess toner particles and paper fibers tend to agglomerate on the interface between the roller and scraper blade. This agglomeration of fibers and toner particles is generally imperceptible or so slight that it is ignored. However, the agglomeration of paper fibers and toner particles may have a serious detrimental affect upon writing quality. For example, the agglomeration of fibers and toner particles leaves unwanted artifacts or ink spots on the edges of the paper.
Accordingly, it is apparent that what is needed is an improved toner applicator which is not subject to toner plating up on the applicator and which can also provide trouble free operation over long periods of time without the need for constant cleaning and maintenance.
What is also needed is a toner applicator which is simple in design, having a longer useful life, and which is capable of providing a uniform flow of toner across the entire surface of a moving recording medium without stain or stop marks which occur through wicking of toner particles in the development meniscus into the paper if the movement of the paper should be interrupted for any reason.
With regard to preventing stop marks and improving image quality, what is also needed is an improved system for articulating a toner applicator between a precise development gap and a separated position away from the surface of the recording medium. It is desirable that a toner applicator articulation system also be capable of substantially instantaneous separation from a recording medium so that the development meniscus is immediately broken and the toner does not wick into the paper. Therefore, what is also needed is an improved articulation system which eliminates the inherent backlash and friction in bearing mounted systems and which would be capable of maintaining a more precise development gap between the surface of a recording medium and a toner applicator for a prolonged period of time.
It would also be desirable to provide a means for eliminating the imperceptible agglomeration of paper fibers, excess toner particles, dust or the like which builds up at the interface between the roller and the scraper blade in prior art multipass electrostatic plotters.